FIG. 1A illustrates an embodiment of a blade server 100, which includes a printed circuit board known as a backplane 102. Blade 104, which in one embodiment is a server, is mechanically attached and electrically coupled to the backplane 102 using socket 106. Blade 104 uses backplane 102 to communicate with other blades also coupled to the backplane through a signal channel 110 within backplane 102. Blade 104 communicates with signal channel 110 through one or more stubs 112 that extend through the thickness of the backplane. For example, blade 104—or, more specifically, circuitry on blade 104—communicates with channel 110 via stub 112.
FIG. 1A illustrates a phenomenon that occurs when a signal is sent or received by blade 104 through stub 112. For example, when a signal originating from blade 104 is launched into stub 112, a first part 118 of the signal travels down the stub and into the channel 110. A second part 116 of the signal, however, travels to the end of stub 112. When the second part 116 of the signal arrives at the end 114 of the stub, it reflects from the end and travels back up stub 112 and at least partly into channel 110. If the data rate of the signal is low and the length of signal channel 112 is small, the reflection of the signal from the stub end usually does not create serious problem, but when the data rate of the signal is high and the length L of signal channel 112 is large, more serious problems begin to occur.
FIGS. 1C and 1D illustrate the problem that occurs with a long signal channel 112 and a high data rate differential signal. FIG. 1B illustrates the differential input signal, measured by the voltage difference between the positive input voltage Vinp and negative input voltage Vinn at the locations shown on blade 204 in FIG. 2A. FIG. 1C illustrates the differential output signal, measured by the voltage difference between the positive output voltage Voutp and negative output voltage Voutn at the locations shown on blade 204 in FIG. 2A. Normally, both FIGS. 1B and 1C should exhibit the well-known “open eye” pattern characteristic of differential signals, and in a perfect communication system, the eye pattern at the input would be perfectly replicated at the output—in other words, the output pattern in FIG. 1C would look exactly like the input pattern in FIG. 1B. Both figures shown that, due to the signal reflection phenomenon discussed above in FIG. 1A, the expected “open eye” pattern becomes somewhat distorted at the input and much more substantially distorted at the output. So much distortion can occur that, as shown in FIG. 1C, the “open eye” pattern closes meaning, essentially, that the signal has become scrambled. The voltage spread has also narrowed, meaning that signal energy has been lost between blades. Among other things, this makes it difficult to decode the signal when it is received.
Current ways of dealing with this problem focus on active solutions that attempt, within the circuitry of the blade receiving a signal, to condition and decode a differential signal. These current solutions, however, are complex and expensive. One passive approach that has been tried is to simply increase the signal power at the input. Unfortunately, though, this approach only affects the voltage spread at the output but does nothing about the closing of the “eye” that occurs between input and output.